Gravitational Waves – The Final Test Of Einstein

In February 2016, LIGO sent waves across the scientific community by announcing the first ever discovery of gravitational waves. Now, the rumour mills are churning again, that LIGO is at it once more. LIGO has, as people have heard, detected gravitational waves from collapse of two neutron stars.

Okay. But what exactly is the fuss about?

In the early 1900’s, the Newtonian idea of gravitation was superceded by Einstein’s General Theory of Relativity. It is a theory which explains the force of gravity among the 4 dimensional fabric of space-time. It accurately makes the predictions which Newtonian gravity does, but also explains other weirder stuff. Gravitational lensing and gravitational redshift are two of the phenomenon that this theory explains. Other than this, GTR also predicted the mathematical existence of Black Holes, a singular point of infinite curvature.

Observations of the cosmos have reaffirmed the position of GTR as the most complete theory of gravitation till date. While it does not blend in with quantum mechanics yet, the predictive accuracy of the theory is astounding. Einstein’s theory also predicts the existence of Gravitational Waves.

Gravitational waves are exactly what they sound like. These waves travel from their source at the speed of light throughout the cosmos. This tells us that the graviton, the hypothetical Gauge Boson of gravitation, is a massless particle. Since gravitation is the weakest of all four fundamental forces, gravitational waves have energies much lower as compared to electromagnetic waves. Only very energetic events, such as black holes colliding, can make waves strong enough to be detected.

It was important to discover them, because they were the only aspect of GTR that wasn’t experimentally verified. And when the team at LIGO announced the first ever direct evidence of gravitational waves, it was the final test that Einstein needed to pass. But that begs the question: if GTR is almost 100 years old, why did it take us so long to make this discovery?

The answer lies, once again, in the weak strength of gravity. While effects of other fundamental forces can be easily observed, gravitational effects require large masses to be seen. It is true that massive bodies release energy in the form of gravitational waves when moving through spacetime, but the energy of such waves is ridiculously small. Even the gravitational waves discovered first by LIGO had a wavelength much smaller than the diameter of a single proton.

Such small wavelengths and energy makes detection of gravitational waves impossible without powerful tools. The way physicists do this is very similar to the experiment conducted my Michelson and Morley, using the Michelson Interferometer.

LIGO Apparatus Arrangement

The LIGO observatories have two mutually perpendicular long tubes, along which a beam of coherent LASER light is incident. These pulses of light are in the same phase initially. Then, both beams are reflected back and forth from two mirrors such that on returning back, they are in opposite phases. Thus, the two beams undergo destructive interference. The photodetector observes no photons.

This is where gravitational waves come in.

If, during the journey of the light, a gravitational wave passes by, the wave actually shrinks and elongates the arms. This means that the phase difference between the two beams is no longer π. There is no destructive interference, and a few photons manage to reach the photodetector. The apparatus is actually sensitive enough to detect a difference of order 10-17 between the waves.

Results from LIGO show the wave passing through spacetime

But there are obstacles in this experiment.

Obstacles

Firstly, there is a lot of noise present due to seismic effects and the Earth itself. This noise interferes with the apparatus and makes detection more difficult. Scientists need to remove this noise to ensure that they are actually witnessing a gravitational wave.

Secondly, the event observed by LIGO was collision of two supermassive black holes. There are no events that can generate such strong gravitational waves, and such collisions are rare.

This concludes the long series of test that Einstein’s theory had to face. With multiple new LIGO observatories opening through the globe, including India, further research is only going to speed up.